MoS2/NiSe2/rGO Multiple-Interfaced Sandwich-like Nanostructures as Efficient Electrocatalysts for Overall Water Splitting

Constructing a heterogeneous interface using different components is one of the effective measures to achieve the bifunctionality of nanocatalysts, while synergistic interactions between multiple interfaces can further optimize the performance of single-interface nanocatalysts. The non-precious metal nanocatalysts MoS2/NiSe2/reduced graphene oxide (rGO) bilayer sandwich-like nanostructure with multiple well-defined interfaces is prepared by a simple hydrothermal method. MoS2 and rGO are layered nanostructures with clear boundaries, and the NiSe2 nanoparticles with uniform size are sandwiched between both layered nanostructures. This multiple-interfaced sandwich-like nanostructure is prominent in catalytic water splitting with low overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and almost no degradation in performance after a 20 h long-term reaction. In order to simulate the actual overall water splitting process, the prepared nanostructures are assembled into MoS2/NiSe2/rGO||MoS2/NiSe2/rGO modified two-electrode system, whose overpotential is only 1.52 mV, even exceeded that of noble metal nanocatalyst (Pt/C||RuO2~1.63 mV). This work provides a feasible idea for constructing multi-interface bifunctional electrocatalysts using nanoparticle-doped bilayer-like nanostructures.


Introduction
Since limited fossil energy is massively consumed and the resulting environmental pollution is becoming increasingly serious, the development of renewable and clean energy is a proven method and has become the future development trend [1][2][3][4]. The high-quality H 2 produced in the hydrogen evolution reaction (HER) at the cathode of a water splitting device can be used as a green energy source, but the oxygen evolution reaction (OER) at the anode faces problems such as multiple reaction steps and slow kinetics [5][6][7]. Many efforts have been made to explore efficient electrocatalysts that can reduce the overpotential of both HER and OER to improve their reaction efficiency. However, the optimal catalysts for HER are Pt or Pt-based nanomaterials, differing from Ir-or Ru-based electrocatalysts for OER [8][9][10][11]. When completely different catalyst materials are used for the cathode and anode of a given water splitting device, the inconsistency of the two electrodes will lead to inefficiencies [12][13][14]. Accordingly, employing different components of nanocatalysts to construct heterogeneous interfaces is the most effective measure to alleviate the above problems, but it still faces a technical bottleneck to further improve the catalytic performance. In addition to searching for bifunctional catalysts that can catalyze both HER and

Synthesis of NiSe 2 /rGO
Graphene oxide (GO) is prepared from pristine graphite flakes using an improved Hummers' method [31]. NiSe 2 /rGO is prepared by an improved facile hydrothermal process [32]. First, 10 mmol NaBH 4 and 8 mmol Se powder are dispersed in 50 mL deionized water with vigorous magnetic stirring for 1 h at room temperature. At the same time, 25 mg GO is dispersed in 20 mL deionized water by ultrasonic treatment for 30 min, and then 4 mmol NiCl 2 ·6H 2 O is added, and the ultrasonic treatment is continued for 30 min. Subsequently, the Se-containing solution and GO-containing suspension are mixed and stirred for 1 h. After that, the mixture is transferred into a 100 mL Teflon-lined autoclave for hydrothermal treatment at 160 • C for 12 h. The resulting product (NiSe 2 /rGO) is collected by filtration, washed with deionized water and ethanol several times, and then dried at 60 • C. For comparison, bare NiSe 2 catalyst is prepared in a similar procedure without adding GO.

Morphological and Structural Characterization
The morphology of MoS 2 /NiSe 2 /rGO is obtained using a focused ion beam scanning electron microscopy (SEM; JCM-6000PLUS, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; JEM-2100, JEOL, Tokyo, Japan). The structure of MoS 2 /NiSe 2 /rGO is analyzed using a powder X-ray diffractometer (XRD; Smart Lab SE, Rigaku, Osaka, Japan). The phase composition and the electronic structures of the elements are investigated via Raman spectroscopy (Raman; Renishaw inVia, Renishaw, New Mills, UK) and X-ray photoelectron spectroscopy (XPS; AXIS Supra, Kratos, Manchester, UK).

Electrocatalysis Measurements
An amount of 20.0 mg of MoS 2 /NiSe 2 /rGO material is dispersed in a 1.0 mL mixed solution of ethanol/deionized water/10% PTFE (1:1:1), ultrasonically treated for 10 min to form a uniform suspension, and then dropped onto clean Ni foam (NF), followed by drying under vacuum at 60 • C. The final mass load of MoS 2 /NiSe 2 /rGO material is calculated to be 4.0 mg cm −2 .
The electrochemical measurements of OER and HER are measured using a threeelectrode workstation (CHI660E, Shanghai Chenhua Instruments, Shanghai, China) at room temperature. The counter, reference, and working electrodes are graphite rods, Ag/AgCl electrodes, and Ni foam electrodes, respectively. The tested potentials are calibrated to reversible hydrogen electrode (RHE) by the Nernst equation: E vs.RHE = E vs.Ag/AgCl + 0.059 pH + 0.197, all linear sweep voltammetry (LSV) curves are corrected with IR compensation (LSV-IR) derived from the electrochemical impedance spectroscopy (EIS) measurements under open circuit voltage [34]. The electrochemical double layer capacitance (C dl ) of the catalyst can be obtained from the linear slope between the current density difference at the cathode and anode and the scan rate [27]. The overall water splitting measurements are measured using a two-electrode system in 1.0 M KOH solution.

Results
The improved Hummers' method is used to prepare rGO with good quality, exhibiting a flat surface and the expected ductility, as shown in Figure S1. Then, the NiSe 2 nanoparticles and MoS 2 layered nanostructures with different ratios are sequentially grown on rGO by a simple hydrothermal method to obtain a series of MoS 2 /NiSe 2 /rGO composites. A schematic diagram of the MoS 2 /NiSe 2 /rGO fabrication process is shown in Figure 1a. Figure S2 shows the SEM images of NiSe 2 and NiSe 2 /rGO, respectively, and it can be seen that the uniformly sized NiSe 2 nanoparticles do not change their size before and after being anchored on rGO. According to the designed plan, MoS 2 is deposited on NiSe 2 /rGO in order to obtain the MoS 2 /NiSe 2 /rGO bilayer-like sandwich nanostructures. It can be seen from Figures 1b and S3  measurements under open circuit voltage [34]. The electrochemical double layer capacitance (Cdl) of the catalyst can be obtained from the linear slope between the current density difference at the cathode and anode and the scan rate [27]. The overall water splitting measurements are measured using a two-electrode system in 1.0 M KOH solution.

Results
The improved Hummers' method is used to prepare rGO with good quality, exhibiting a flat surface and the expected ductility, as shown in Figure S1. Then, the NiSe2 nanoparticles and MoS2 layered nanostructures with different ratios are sequentially grown on rGO by a simple hydrothermal method to obtain a series of MoS2/NiSe2/rGO composites. A schematic diagram of the MoS2/NiSe2/rGO fabrication process is shown in Figure  1a. Figure S2 shows the SEM images of NiSe2 and NiSe2/rGO, respectively, and it can be seen that the uniformly sized NiSe2 nanoparticles do not change their size before and after being anchored on rGO. According to the designed plan, MoS2 is deposited on NiSe2/rGO in order to obtain the MoS2/NiSe2/rGO bilayer-like sandwich nanostructures. It can be seen from Figures 1b and S3   The structure of MoS2/NiSe2/rGO is characterized more finely by TEM, and it is clearly seen in Figure 1c that MoS2 and rGO have a high-quality layered structure with The structure of MoS 2 /NiSe 2 /rGO is characterized more finely by TEM, and it is clearly seen in Figure 1c that MoS 2 and rGO have a high-quality layered structure with NiSe 2 nanoparticles uniformly sandwiched between both, forming a well-defined bilayerlike sandwich nanostructure. Figure 1d shows a typical local structure at the boundary, and Figure 1 [18,35]. The STEM-energy dispersive spectrometer (EDS) elemental mapping of MoS 2 /NiSe 2 /rGO in Figure 1e confirms the presence of element C and shows its uniform distribution in the layered sandwich-like nanostructure. It is also noted that the distributions of Mo, S, Ni, and Se elements are almost identical, indicating not only that MoS 2 and NiSe 2 are uniformly distributed on rGO but also that the NiSe 2 nanoparticles are covered in the MoS 2 layered nanostructure.
Subsequently, the structural characteristics, phase composition, and element analysis of the prepared samples are analyzed by XRD, Raman, and XPS spectra, respectively. Figures S1b and S5a,b exhibit the XRD spectra of rGO, NiSe 2 , and MoS 2 , respectively. As shown in Figure 023), and (321) crystal plane of NiSe 2 , respectively, which are well consistent with the standard crystal of NiSe 2 (PDF No. 65-5016), indicating that NiSe 2 exhibits excellent crystallinity in all three samples [35]. In contrast, none of the diffraction peaks of MoS 2 appear, which is due to the poor crystallinity of MoS 2 obtained by the hydrothermal method [36]. Raman spectroscopy is used to further determine the composition of MoS 2 /NiSe 2 /rGO. As shown in Figures 2b and S6, the Raman characteristic peaks of the MoS 2 /NiSe 2 /rGO and pure MoS 2 samples at 375 and 401 cm −1 belong to the E 1 2g and A 1g modes of the semiconductor phase (2H) MoS 2 , representing the in-plane Mo-S bond and the out-of-plane S atom vibration mode, respectively [37,38]. In addition, the characteristic peaks of NiSe 2 are also displayed at 148, 208, and 235 cm −1 , while the peaks at 1358 and 1588 cm −1 correspond to the D band and G band of rGO, respectively [39,40]. The Raman results confirm that the 2H-phase-dominated MoS 2 nanosheets are on the NiSe 2 /rGO surfaces. As shown in the XPS spectrum of S 2p (Figure 2d), pure MoS2 has only two typical peaks corresponding to S 2p1/2 and S 2p3/2. In contrast, the SMo-S 2p1/2 and SMo-S 2p3/2 of the MoS2/NiSe2/rGO have a negative offset, implying the occurrence of electron transfer. At the same time, two peaks are separated at 161.6 and 162.8 eV, corresponding to SNi-S 2p1/2 and SNi-S 2p3/2, respectively. Compared with pure MoS2, the formation of heterostructure caused a slight negative shift in the energy band position of S (~0.16 eV), which is due to the synergistic effect between NiSe2 and molybdenum disulfide [44]. In addition, the positive displacement of Se 3d in Figure S7c fully proves this point. The XPS results confirmed XPS is used to examine the molecular structure and atomic valence states of the materials [41]. The presence of Mo, S, Ni, and Se in the MoS 2 /NiSe 2 /rGO material is confirmed from the full XPS spectrum ( Figure S7a), but the Ni content on the surface of the sample is relatively low ( Figure S7b). Figure 2c,d present the high-resolution XPS spectra of Mo 3d and S 2p in pure MoS 2 and MoS 2 /NiSe 2 /rGO, respectively. As shown in Figure 2c, the fitting peaks of the pure MoS 2 at 232.2, 229, and 226.1 eV correspond to Mo 4+ 3d 3/2 , Mo 4+ 3d 5/2 , and S 2s, respectively, while the high binding energy peak of the Mo 3d (235.5 eV) corresponds to the MoO 3 , which may result from the oxidation of the sample in the air [42]. Compared with pure MoS 2 , both the peaks of Mo 4+ 3d 3/2 and Mo 4+ 3d 5/2 in MoS 2 /NiSe 2 /rGO are slightly shifted to lower binding energies (~0.35 eV), which indicates that there is a significant electron transfer between NiSe 2 and MoS 2 . In addition, the peak at 232.8 eV is attributed to Mo-Se, which also indicates the formation of a heterogeneous interface between MoS 2 and NiSe 2 , where some of the Se atoms replace the S atoms [43].
As shown in the XPS spectrum of S 2p (Figure 2d), pure MoS 2 has only two typical peaks corresponding to S 2p 1/2 and S 2p 3/2 . In contrast, the S Mo-S 2p 1/2 and S Mo-S 2p 3/2 of the MoS 2 /NiSe 2 /rGO have a negative offset, implying the occurrence of electron transfer. At the same time, two peaks are separated at 161.6 and 162.8 eV, corresponding to S Ni-S 2p 1/2 and S Ni-S 2p 3/2 , respectively. Compared with pure MoS 2 , the formation of heterostructure caused a slight negative shift in the energy band position of S (~0.16 eV), which is due to the synergistic effect between NiSe 2 and molybdenum disulfide [44]. In addition, the positive displacement of Se 3d in Figure S7c fully proves this point. The XPS results confirmed the strong interfacial electronic interactions in the heterostructure and the electrons transfer from NiSe 2 to MoS 2 , which would optimize the electronic structure of NiSe 2 and MoS 2 and thus enhance the activity of OER and HER.
The OER electrocatalytic performance of MoS 2 /NiSe 2 /rGO in an alkaline solution (1 M KOH) is first investigated (Figure 3). To better demonstrate the effect of multiple interfaces on performance, three heterogeneous interfaces (MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO) consisting of three constituent units are used as the samples to be tested under the same conditions. Meanwhile, the properties are tested for NiSe 2 , NF, and RuO 2 , and different precursor ratios of MoS 2 /NiSe 2 -1, MoS 2 /NiSe 2 -2, MoS 2 /NiSe 2 /rGO-1, and MoS 2 /NiSe 2 /rGO-2 are investigated, as shown in Figure S8. From the iR-compensated LSV curves (Figure 3a), it is obvious that MoS 2 /NiSe 2 /rGO has the optimum performance among the four electrocatalysts. When the current density is 20 mA cm −2 , the overpotential of MoS 2 /NiSe 2 /rGO is only 277 mV, which is 22, 51, and 113 mV lower than that of MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO, respectively. For a comprehensive comparison of the LSV performance of the four catalysts, whose overpotentials at two additional current densities are also identified. As shown in Figure 3b, although the overpotentials of the four catalysts increase gradually with the current density, the overpotential of MoS 2 /NiSe 2 /rGO is consistently the lowest among them. The overpotential of MoS 2 /NiSe 2 /rGO is surprisingly 123 mV lower than that of MoS 2 /rGO when the current density is 100 mA cm −2 , and the relevant values are shown in Table S2. The results fully demonstrate that the NiSe 2 nanoparticles sandwiched between the two layered structures can effectively optimize the overpotential of MoS 2 /rGO. Furthermore, multiple interfacial nanostructures can greatly improve the contribution of a single interface to performance, and this new multi-interfacial nanostructure constructed by the three composites acting together can facilitate the OER performance [44,45].
The enhanced OER kinetics of MoS 2 /NiSe 2 /rGO is confirmed by the smaller Tafel slope (107 mV dec −1 ) compared to those of MoS 2 /NiSe 2 (157 mV dec −1 ), NiSe 2 /rGO (110 mV dec −1 ), and MoS 2 /rGO (126 mV dec −1 ), as shown in Figure 3c. To further validate the electron transfer dynamics of OER, EIS are also performed on four electrocatalysts. From the corresponding Nyquist plots in Figure 3d, it can be seen that MoS 2 /NiSe 2 /rGO exhibits significantly lower charge transfer resistance compared to NiSe 2 /rGO, MoS 2 /NiSe 2 , and MoS 2 /rGO, indicating faster electron transfer on MoS 2 /NiSe 2 /rGO, and also suggesting that the multiple interfaced sandwich-like nanostructure can significantly improve the conductivity of the electrocatalyst. The C dl values are proportional to the electrochemical active surface area (ECSA) and can be estimated by performing cyclic voltammetry (CV) curves at different scan rates in a non-faradic potential range, which is usually used to further evaluate the OER catalytic activity [11]. As shown in Figure 3e, the C dl values of the four electrocatalysts are calculated from the slopes of the linear fits of the CV curves ( Figure S9), in which MoS 2 /NiSe 2 /rGO has the largest electrical double-layer capacitance value, denoting a larger electrochemically active surface area. Durability is also an important indicator to assess the performance of the electrocatalyst. The chronopotential measurement curve of MoS 2 /NiSe 2 /rGO at a constant current density of 10 mA cm −2 is obtained for continuous 20 h (Figure 3f). Compared to the initial potential of 1.46 V, there is only a slight elevation of 30 mV in the potential after 20 h of continuous oxygen generation. This indicates that the as-prepared MoS 2 /NiSe 2 /rGO not only has excellent catalytic activity, but also has outstanding catalytic durability. MoS2/NiSe2/rGO is surprisingly 123 mV lower than that of MoS2/rGO when the current density is 100 mA cm −2 , and the relevant values are shown in Table S2. The results fully demonstrate that the NiSe2 nanoparticles sandwiched between the two layered structures can effectively optimize the overpotential of MoS2/rGO. Furthermore, multiple interfacial nanostructures can greatly improve the contribution of a single interface to performance, and this new multi-interfacial nanostructure constructed by the three composites acting together can facilitate the OER performance [44,45]. The enhanced OER kinetics of MoS2/NiSe2/rGO is confirmed by the smaller Tafel slope (107 mV dec −1 ) compared to those of MoS2/NiSe2 (157 mV dec −1 ), NiSe2/rGO (110 mV dec −1 ), and MoS2/rGO (126 mV dec −1 ), as shown in Figure 3c. To further validate the electron transfer dynamics of OER, EIS are also performed on four electrocatalysts. From the corresponding Nyquist plots in Figure 3d, it can be seen that MoS2/NiSe2/rGO exhibits significantly lower charge transfer resistance compared to NiSe2/rGO, MoS2/NiSe2, and MoS2/rGO, indicating faster electron transfer on MoS2/NiSe2/rGO, and also suggesting that the multiple interfaced sandwich-like nanostructure can significantly improve the conductivity of the electrocatalyst. The Cdl values are proportional to the electrochemical active surface area (ECSA) and can be estimated by performing cyclic voltammetry (CV) curves at different scan rates in a non-faradic potential range, which is usually used to further evaluate the OER catalytic activity [11]. As shown in Figure 3e, the Cdl values of the four electrocatalysts are calculated from the slopes of the linear fits of the CV curves ( Figure S9), in which MoS2/NiSe2/rGO has the largest electrical double-layer capacitance value, denoting a larger electrochemically active surface area. Durability is also an important indicator to assess the performance of the electrocatalyst. The chronopotential measurement curve of MoS2/NiSe2/rGO at a constant current density of 10 mA cm −2 is obtained for continuous 20 h (Figure 3f). Compared to the initial potential of 1.46 V, there is only a slight elevation of 30 mV in the potential after 20 h of continuous oxygen genera- The HER performance of the samples is evaluated in the same electrolyte (1.0 M KOH) at a scan rate of 5 mV s −1 (Figures 4 and S10). Figure 4a shows the LSV curves of MoS 2 /NiSe 2 /rGO, MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO with iR-compensation, and it can be seen that the curves are smooth with a uniform trend, which proves that all four catalysts have HER activity. However, the overpotential of the four catalysts differed greatly at the same current density, as marked in the purple line in Figure 4a at a current density of 10 mA cm −2 , and MoS 2 /NiSe 2 /rGO has the lowest overpotential among the four catalysts, indicating that it has the favorable HER activity. For a more comprehensive assessment of the intrinsic reaction kinetics of the catalysts, the overpotentials at different current densities are visualized in Figure 4b as histograms. It can be seen that MoS 2 /NiSe 2 /rGO exhibits the lowest overpotential regardless of the determined current density and is the best among four samples in HER. Even at a high current density of 100 mA cm −2 , its overpotential is only 223 mV, which is better than 241 mV of MoS 2 /NiSe 2 , 301 mV of NiSe 2 /rGO, and 374 mV of MoS 2 /rGO. sessment of the intrinsic reaction kinetics of the catalysts, the overpotentials at different current densities are visualized in Figure 4b as histograms. It can be seen that MoS2/NiSe2/rGO exhibits the lowest overpotential regardless of the determined current density and is the best among four samples in HER. Even at a high current density of 100 mA cm −2 , its overpotential is only 223 mV, which is better than 241 mV of MoS2/NiSe2, 301 mV of NiSe2/rGO, and 374 mV of MoS2/rGO. The calculated Tafel slopes of the four catalysts are calculated to be 73, 79, 105, and 149 mV dec −1 , respectively, as shown in Figure 4c. The lower Tafel slopes of MoS2/NiSe2/rGO and MoS2/NiSe2 prove that the combination of MoS2 and NiSe2 helps to improve the reaction rate and kinetics [46]. The corresponding Nyquist plots shown in Figure 4d are similar to the OER test results. Compared with MoS2/NiSe2, NiSe2/rGO, and MoS2/rGO, MoS2/NiSe2/rGO exhibits lower charge transfer resistance, indicating that MoS2/NiSe2/rGO has superior electron transport capability. In addition, the Cdl is again used to assess the HER catalytic activity of the catalysts. Figures 4e and S11 reveal that the fitted line slopes of 38.13, 21.11, 23.76, and 5.25 mF cm −2 for MoS2/NiSe2/rGO, MoS2/NiSe2, NiSe2/rGO, and MoS2/rGO, respectively, suggesting that the multiple interfaced sandwich-like nanostructure provides more effective active sites than the other three catalysts [47]. Of these, more active sites might be due to the effect of the unique heterogeneous interface of MoS2/NiSe2/rGO. Furthermore, the HER electrochemical stability of MoS2/NiSe2/rGO is also evaluated by the chronopotentiometry measurement curve The calculated Tafel slopes of the four catalysts are calculated to be 73, 79, 105, and 149 mV dec −1 , respectively, as shown in Figure 4c. The lower Tafel slopes of MoS 2 /NiSe 2 /rGO and MoS 2 /NiSe 2 prove that the combination of MoS 2 and NiSe 2 helps to improve the reaction rate and kinetics [46]. The corresponding Nyquist plots shown in Figure 4d are similar to the OER test results. Compared with MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO, MoS 2 /NiSe 2 /rGO exhibits lower charge transfer resistance, indicating that MoS 2 /NiSe 2 /rGO has superior electron transport capability. In addition, the C dl is again used to assess the HER catalytic activity of the catalysts. Figure 4e and Figure S11 reveal that the fitted line slopes of 38.13, 21.11, 23.76, and 5.25 mF cm −2 for MoS 2 /NiSe 2 /rGO, MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO, respectively, suggesting that the multiple interfaced sandwichlike nanostructure provides more effective active sites than the other three catalysts [47]. Of these, more active sites might be due to the effect of the unique heterogeneous interface of MoS 2 /NiSe 2 /rGO. Furthermore, the HER electrochemical stability of MoS 2 /NiSe 2 /rGO is also evaluated by the chronopotentiometry measurement curve (Figure 4f). After 20 h of continuous testing, the overpotential increases by only 15 mV at a constant current density of 10 mA cm −2 . Moreover, MoS 2 /NiSe 2 /rGO shows negligible catalytic degradation, confirming its excellent electrochemical stability.
In view of the excellent OER and HER catalytic behaviors of the MoS 2 /NiSe 2 /rGO catalyst discussed above, it is further served as a bifunctional catalyst and assembled into a two-electrode test system to investigate its overall water splitting performance in 1.0 M KOH electrolyte. The schematic diagram of all-water electrolysis in the double-electrode test system is shown in Figure 5a. At the cathode, the electrons transferred to the electrolyte combine with water to produce H 2 gas, while the generated hydroxide ions are transferred to the anode and oxidized to release O 2 gas. Polarized electrons are released at the anode so that the hydroxide is oxidized in the solution to release O 2 gas. The two-electrode test device is shown in Figure 5b. In the LSV curves with a scan rate of 5 mV s −1 shown Nanomaterials 2023, 13, 752 9 of 13 in Figure 5c, MoS 2 /NiSe 2 /rGO||MoS 2 /NiSe 2 /rGO exhibits excellent catalytic activity, affording a current density of 10 mA cm −2 at 1.52 V, while Pt/C||RuO 2 requires the same current density at 1.63 V. The inset is a photograph of the experimental device, clearly showing the high density of bubbles on the surface of both electrodes. The H 2 and O 2 generated from the MoS 2 /NiSe 2 /rGO catalyzed overall water-splitting are collected quantitatively by the drainage method. As shown in Figure 5d, the volume ratio of collected H 2 to O 2 is 2.05:1, close to the theoretical value of 2:1. Taking into account that the airtightness of the equipment produces errors, it can be seen that the Faraday efficiency of the overall water splitting is almost 100% [48]. The stability test results of MoS 2 /NiSe 2 /rGO at a constant current density of 10 mA cm −2 are shown in Figure 5e. It can be seen from the chronopotential curve that MoS 2 /NiSe 2 /rGO||MoS 2 /NiSe 2 /rGO features extremely outstanding stability, which maintains the output voltage almost constant at 1.65 V throughout the continuous 25 h of the overall water splitting performance. In general, MoS 2 /NiSe 2 /rGO is proven to trigger the overall water splitting with favorable efficiency and stability. Compared with the recently reported advanced electrocatalysts, MoS 2 /NiSe 2 /rGO demonstrates outstanding performance (Table S1), indicating that the prepared sandwich nanostructure is an excellent overall water splitting bifunctional electrocatalyst [16,[49][50][51][52][53].

Conclusions
In summary, MoS2/NiSe2/rGO with multiple interfaced sandwich-like nanostructures consisting of NiSe2 nanoparticles sandwiched between layered MoS2 and rGO are successfully prepared by a simple and convenient two-step hydrothermal method. Compared with similar catalysts, MoS2/NiSe2/rGO has better electrocatalytic performance, even OER and overall water splitting surpass those of the noble metals, which are attributed to the The outstanding OER, HER, and overall water splitting performance of MoS 2 /NiSe 2 /rGO, where OER and overall water splitting even surpasses those of noble metals (Figures 5 and S8), are mainly attributed to the highly advantageous multiple interfaced sandwich-like nanostructure of the NiSe 2 nanoparticles sandwiched between two different layered structures (MoS 2 and rGO). The DFT calculations confirm this point of view ( Figure S12). Due to the smaller size of the intercalated NiSe 2 nanoparticles, this unique nanostructure can simultaneously construct three different interfacial relationships: MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO, and the synergistic effect of the three interfaces can dramatically facilitate the rapid and smooth electrocatalytic reaction. Moreover, the NiSe 2 nanoparticles are sandwiched between both layers, making them less tightly packed and contributing to the rapid transport of the gases generated by the reaction, enhancing the reaction dynamics, as evidenced by the Tafel curves. In addition, compared with the ordinary nanoparticles, both layered nanostructures that supported nanoparticles are more helpful in maintaining a robust nanostructure for excellent stability. This design approach can offer new ideas for obtaining efficient catalysts with structural stability.

Conclusions
In summary, MoS 2 /NiSe 2 /rGO with multiple interfaced sandwich-like nanostructures consisting of NiSe 2 nanoparticles sandwiched between layered MoS 2 and rGO are successfully prepared by a simple and convenient two-step hydrothermal method. Compared with similar catalysts, MoS 2 /NiSe 2 /rGO has better electrocatalytic performance, even OER and overall water splitting surpass those of the noble metals, which are attributed to the efficient interfacial relationship between MoS 2 /NiSe 2 , NiSe 2 /rGO, and MoS 2 /rGO. The overpotential of MoS 2 /NiSe 2 /rGO||MoS 2 /NiSe 2 /rGO in overall water splitting is only 1.52 mV, which is much lower than that of the noble metal electrocatalyst Pt/C||RuO 2 (1.63 mV). Additionally, there is almost no loss of performance in the long-term stability test, implying its good stability. This multi-interface idea can open new avenues for the design of efficient bifunctional overall water catalysts.  Table S1. Comparison of electrocatalytic performance of different catalysts in 1 M KOH electrolyte; Table S2. Comparison of the electrocatalytic activity of MoS 2 /NiSe 2 /rGO with MoS 2 /rGO, NiSe 2 /rGO in 1 M KOH electrolyte. References [16,[49][50][51][52][53][54][55][56][57] are cited in the Supplementary Materials.